In many applications involving treatment of polymer material there is a need of a fast and uniform, simultaneous heating of the entire polymer material. There are many ways of heating the polymer material, see the discussion below regarding cross-linking, but most of them have the common feature that they involve a heating of the surface of the material and conduction through the material. The heating thus will be slow and non-uniform since polymer materials generally are poor conductors in respect of heat.
Cross-linking, that is, the formation of spatial networks or cross-links in polymers such as polyethylene is nowadays generally carried out in accordance with two main principles.
According to a first main principle the formation of the spatial network is achieved chemically by means of functional groups, that is, chemically reactive side groups on the polyethylene chain take part in a condensation process which causes cross-links. Trismethoxy-vinylsilane is used in this process and for this reason the process is called the silane method.
According to another main principle the cross-linking is achieved by means of radicals which either may be formed by direct action of radiation energy on the polymer chain, for instance by means of irradiation with an electron beam, or with UV-radiation, or which may be formed by means of additives, cross-linking agents, such as organic peroxides or azo-compounds. In the latter case the cross-linking reaction normally is triggered by means of heat. The invention inter alia relates to this latter type of cross-linking by means of heat.
SE-B-324 450 discloses a process in which the polymer which is to be cross-linked is mixed with a cross-linking agent such as an organic peroxide before the extrusion and during or after the extrusion is heated so as to initiate the formation of a spatial network. This process has disadvantages because of the specific design thereof, primarily in the continuous manufacture of products like tubes, particularly in that the speed of manufacture will be very low.
Methods based on the transfer of heat from the tool generally have the advantage that all the peroxide will be used since the peroxide does not have time to evaporate. However, very long tools are then needed in order to allow the temperature to be raised in a degree which is sufficient to trigger the cross-linking. A consequence of this is that there is a high pressure drop in the tool, resulting in the necessity of high extrusion pressures, which in turn entails that the extruder must meet high standards--normally a so-called ram-extruder must be used. A result of long channels also is frictional problems in the tool. Attempts to solve the frictional problems have involved coating the tools with teflon and combining screw extruders with tools for instance provided with coextrusion, see e.g. WO 94/21441. In these processes, however, the coextruded surface layers may have to be removed after the extrusion.
In one process (called PEXEP) described in the Finnish laid-open specification 94106, which is substantially faster than the preceding process, an extruded tube of polyethylene is heated by direct contact with heated wheels in order to initiate the cross-linking. The heating here is mainly carried out by heating the surface and consequently involves conduction through the material. It should be noted that polymers like polyethylene are poor conductors in respect of heat. The disadvantages associated with this process are that the dimensional stability and the surface finish are less good. The PEXEP-process also has the disadvantage that the peroxide may evaporate from the surface and that for this reason it may be difficult to achieve an uniform cross-linking throughout the entire tube wall.
In another process the heating is carried out with a liquid in the form of a salt bath. The heat transfer from liquid to polymer is relatively good, but the problems with a surface finish which is less good and the poor heat conductivity of polyethylene remain.
In Plastics Extrusion Technology, Chapter 15, Cross-linking of plastics after extrusion, pp 499-500 (Hanser Publishers, Munich 1988), it is described how comparatively thick insulation on cables can be cross-linked contactlessly by utilizing vertical heating devices in the form of tubes containing hot vapour, without any risk for deformation of the insulation due to the influence of gravity. Furthermore a deformation of the surface of the insulation of the cable caused by contact with the walls of the vapour tube is avoided. The heating is however carried out utilizing conduction and for this reason must be performed under a long period of time, especially if the materials are thick, since the polymer, as mentioned above, is a poor conductor in respect of heat. The maximum temperature which may be achieved is furthermore low, about 225.degree. C., which further increases the the cross-linking time and thereby the length of the cross-linking zone. A comparatively long part of the cable must be cantilevered in the cross-linking zone. It would thus seem that one condition is that the polymer is carried or supported by a core (the cable) in order to diminish any deformation or yielding caused by gravitation.
GB-A-1 562 814 discloses a continuous process for the manufacture and the cross-linking of cellular plastic made of polyethylene in which the polyethylene initially is mixed with peroxide and cellforming agents, whereupon, in one alternative, cross-linking in a first step is initiated by means of a limited heating obtained by means of infrared radiation after which the cross-linking is carrried out by means of the exothermic heat. In a second step the plastic is expanded by additional heating by means of hot air which simultaneously functions as a supporting air cushion.
Hiroshi Nishimura--Shunichi Takai, Corrosion protection of submarine pipeline with a wrap-around heat shrinkable tube, pp. 55-66, UK Corrosion '83; Conference; Ed.: Institution of Corrosion Science & Technology; National Association of Corrosion Engineers; Birmingham Nov. 15-17, 1983 discloses the initiation of cross-linking in polyethylene in shrinkable tubes by means of infrared radiation, the infrared radiation being adapted so as to consist of the wave lengths which substantially entirely are absorbed by polyethylene. Since the radiation essentially is absorbed, mainly the surface layer of the polymer will be heated and the remaining part of the polymer will be heated by means of conduction. The process thus is mainly suited for thin materials or manufacture at low speeds. A disadvantage with this process is the risk for overheating in the surface layer.